1. Field of the Invention
The present invention relates generally to multi-mode and single mode optical fibers. Specifically, the invention relates to a novel method for drawing and inserting a core fiber into a cladding material to form an optical fiber preform having a high quality core-clad interface for use in fabricating a low loss optical fiber.
2. Description of the Related Art
Single mode optical fibers that transmit both visible and infrared energy are desirable for use in long distance communications systems. Minimization of transmission loss is particularly important in preparing optical fibers for use in long distance applications. Conventional methods for fabricating optical fibers involve casting glass melts into glass preforms which are then drawn into optical fibers.
Preforms comprising multicomponent fluoride glasses are typically used to fabricate low loss optical fibers, particularly, single mode fibers which can carry signals over distances of several thousand kilometers without the need for repeaters that regenerate the signal. Multicomponent fluoride glasses are particularly susceptible to crystallite formation since these glasses have significantly lower theoretical transparencies than silica glasses and exhibit low viscosities at their liquidus temperatures. Furthermore, the melts of multicomponent glasses are reactive with ambient gases and crucible materials which increases their susceptibility to crystallization and contamination.
Conventional methods for making optical fiber preforms expose the core and cladding materials to temperatures exceeding crystallization temperatures during the addition of the core material to the cladding material. These methods involve cooling a cladding glass melt and a subsequent reheating of the cladding glass to temperatures greater than the crystallization temperature upon the addition of a core glass melt to the cladding glass. Conventional methods for forming optical fiber preforms such as, for example, suction casting, build-in-casting, rotational casting and rod-in-tube casting are described in Chapter 5 of Fluoride Glass Fiber Optics, Academic Press, Inc., edited by Ishivar D. Aggarwal and Grant Lu, pp. 223-227 (1991). See also, for example, U.S. Pat. Nos. 4,793,842, 5,106,400 and 5,160,521 the disclosures of which are incorporated herein by reference.
Preforms cast by conventional methods must be modified before being drawn into single mode fibers having desired core and cladding diameters. Examples of such modifications include stretching procedures at high temperatures which further increase crystallite formation and multiple jacketing procedures.